The present invention relates to a laser system for materials processing, with a device serving to produce a linearly polarized laser beam and with at least one delay plate for producing a polarized laser beam that has a rotating polarization vector and in particular an elliptically or circularly polarized laser beam.
In the processing of materials, for instance in cutting or welding with a linearly polarized laser beam, the processing result obtained depends on the beam vector. To minimize or eliminate this dependency, prior art laser systems are equipped with devices serving to produce a polarized laser beam with a rotating polarization vector, and preferably an elliptical or circular polarization of the laser beam. A laser system of that type, incorporating a xcex/4 plate as a delay surface for the circular polarization of a linearly polarized laser beam has been described in EP-B-0 591 541. In that design, the polarization plane of the linearly polarized laser beam impinging on the xcex/4 delay plate is inclined at an angle of 45xc2x0 relative to the reflection plane of the delay plate. This 45xc2x0 angle is obtained by means of a mirror assembly preceding the xcexd/4 plate in the light path of the laser beam inside the prior-art laser resonator.
Prior art laser systems for materials processing are also equipped with devices for beam forming and in particular for expanding the laser beam produced; these are in the form of so-called xe2x80x9cbeam telescopesxe2x80x9d. Their purpose is to keep the angle of divergence of the laser beam reasonably small for a relatively long laser beam path. In this fashion it is possible, with minimal losses, to allow the laser beam to impinge on the optical beam-focussing element typically positioned near the processing point on the workpiece and to focus it on the latter. A laser system employing a beam telescope is described for instance in EP-A-0 428 734.
With respect to the above-mentioned prior-art designs, it is the object of this invention to minimize the number of optical elements needed in laser systems configured for materials processing and incorporating a laser beam forming device.
A specific object is to provide a laser resonator in combination with a novel beam forming device which shapes the laser beam and imports a rotating polarization vector thereto.
It has now been found that the foregoing and related objects can be readily attained by a laser beam generating assembly for producing a linearly polarized laser beam with a rotating polarization vector comprising a laser resonator for generating a linearly polarized laser beam, and a novel laser beam forming device. The beam forming device includes a delay plane having the plane of its reflective surface oriented at an angle of 45xc2x0 to the vertical, and a plurality of beam-forming optical elements in the laser beam path prior to the delay plate. The optical elements have reflective surfaces angularly oriented to orient the polarization plane of the laser beam at an angle of 45xc2x0 to the plane of incidence defined by the normal vector to the plane of the reflective surface of the delay plate and the oriented laser beam incident thereon, whereby the laser plate redirects the beam in a vertical direction and imparts a rotating polarization vector thereto.
In the present invention, at least one beam-forming optical element also doubles as a delay plate and at least one beam-forming optical element is utilized for the defined alignment of the polarization plane of the linearly polarized laser beam relative to the delay plate. Thus, only a relatively small number of optical elements need to be interpositioned in the light path of this type of laser system. This fact offers a number of advantages. Laser systems using this invention can be relatively small in design and their correspondingly simple construction enhances their cost effectiveness. Given the fact that laser beam forming or polarizing optical elements inherently cause light-energy losses and that optical elements of the type in question always constitute potential sources of error in the proper path alignment of the laser beam, the reduced number of optical elements in the design of this invention also entails reduced attendant energy losses and error sources in the beam alignment. The dual-purpose optical elements offer the same functional properties as those conventional optical elements whose functionalities they combine.
The characteristic features of patent claims 3 and 5 take into account the fact that, in the case of materials-processing laser systems according to this invention, it is typically laser beams with a relatively high output energy that must reach the workpiece.